The present invention relates to a control rod for a boiling water reactor (BWR). More particularly, it relates to a control rod of a neutron-absorbing material which encloses a moderating substance.
A core in a nuclear reactor normally contains several hundred fuel assemblies. These are arranged vertically in the core and have an at least substantially square cross section. Each fuel assembly comprises a bundle of fuel rods which are surrounded by a fuel channel. The fuel channel is open at both ends allowing the coolant of the reactor to flow through the fuel assembly. The core is immersed into water which serves both as coolant and as neutron moderator.
The core also includes a large number of control rods, each of which comprises four vertically arranged absorber blades forming a right-angled cross. The fuel assemblies are normally arranged in a regular lattice. Each fuel assembly is included in two rows of fuel assemblies which are perpendicular to each other. The control rods are normally arranged with each one of their absorber blades between two fuel assemblies located in the same row such that each control rod together with four fuel assemblies, arranged around the blades of the control rod, forms one unit. By moving the control rods into and out of the core, the power distribution of the core can be controlled.
Conventional control rods for boiling water reactors are composed of four elongated absorber blades which are arranged in a cruciform. The absorber blades are generally of stainless which are steel and provided with a large number of bored holes or tubes filled with a powder of boron carbide (B4C), acting as neutron absorber.
When a control rod is inserted into the core in the reactor, the neutron absorber is irradiated by neutrons and successively loses neutron absorption capacity. The control rod is therefore replaced after being used for a predetermined operating period. It is true that boron carbide has a great ability to absorb neutrons, but it has disadvantages. Since the boron carbide is in a powdered state, it must be encapsulated into the absorber blades. When the boron carbide absorbs neutrons, it swells, which causes great mechanical stresses on the cladding. Control rods with boron carbide as absorber material therefore have a relatively short service life.
A number of different proposals for new control rods with longer service lives have been produced, one example being disclosed in the patent specification U.S. Pat. No. 5,034,185. Common to these proposals is that the neutron absorber consists of a material with a comparatively long life, for example hafnium. A further advantage with hafnium, in relation to the boron carbide is its good corrosion properties, which means that hafnium withstands contact with the cooling water and need not be encapsulated. On the other hand, hafnium is not as good a neutron absorber as boron carbide, so in order to obtain an equally effective control rod the quantity of absorber material needs to be increased. One problem with hafnium is that it is expensive and has a high density, which entails an increase in both the weight and the cost of control rods manufactured of this material.
The probability that a neutron will be absorbed increases the lower its energy is. Thus, a fast neutron has a very small probability of being absorbed. In the patent specification U.S. Pat. No. 4,876,060 a control rod is described in which the absorber blades comprise absorber plates of hafnium, which are arranged such that a channel filled with water is formed between them. The water moderates (slows down) the neutrons which are so fast that they are not absorbed the first time they pass through the absorber plate. The fast neutrons are slowed down by collisions with the hydrogen atoms of the water and will be absorbed with a higher probability the next time they pass through one of the absorber plates. By using a moderator between the absorber plates, the absorption capacity of the control rod increases, which means that the absorber plates can be made thinner. In this way, the consumption of the expensive and heavy absorber material can be reduced. This technique is common knowledge and usually referred to as a neutron trap.
The control rod described above is composed of four absorber blades, which are arranged in a cruciform and fixed to a central supporting rod, a so-called center rod. The control rod also has an external shroud of, for example, stainless steel. The center rod consists of a rod whose length corresponds to the length of the absorber blades in the longitudinal direction of the control rod. A disadvantage of the control rod described above is that the volume of the moderator is limited by the centre rod and the shroud. With a different embodiment of the control rod, the volume of the moderator can be increased and the absorption capacity of the control rod be further improved.
It is an object of the invention to propose a control rod which
has a high neutron absorption capacity,
has a long service life,
is simple and inexpensive to manufacture,
does not weigh too much.
The invention relates to a control rod comprising a top piece and a bottom piece between which an absorber part is arranged. The absorber part comprises a central part and four absorber blades extending from the central part. The absorber part comprises a channel which comprises the central part and which extends along the length of the whole absorber part. The channel is surrounded by walls which comprise a neutron-absorbing material with a long service life, for example hafnium. The channel is at least partially filled with a moderator, for example light water.
By arranging a moderator also in the central part of the control rod, an increase of the total volume of the moderator and a large coherent moderator volume are obtained. A large coherent moderator volume slows down the neutrons more than a plurality of smaller moderator volumes. This is due to the fact that the neutrons are able to move a longer distance in the larger volume and can, therefore, be slowed down to a greater extent. This means a great improvement of the efficiency of the control rod compared with previous solutions where moderator water is only present in a number of individual channels in the absorber blades and where the center is occupied by a central rod.
The control rod is simple in construction and is therefore inexpensive to manufacture. The weight and cost of the control rod can be weighed against its service life and its efficiency and is determined in a flexible way by the thickness of the absorber plate.
The control rod becomes flexible in a direction perpendicular to the longitudinal direction of the control rod. This provides good properties from a seismic point of view, and the flexibility also prevents the control rod from being stuck in the core, because of bowed control rods or fuel assemblies or other similar reasons, when it is to be moved out of and into the core.
During manufacture of the absorber part it may undergo a heat treatment such that the hafnium alloy has completely or partially been transformed from xcex1-phase to xcex2-phase and thereafter rapidly cooled to xcex1-phase.